For several new LED based lighting systems, light guides are used. Such a light guide, typically a flat or curved piece of transparent plastic or glass, usually serves two purposes. It guides light from a light source, e.g. one or more light emitting diodes (LEDs), to a desired spot, and helps it mixing the colours from individual red, green and blue LEDs. It is especially important with the mixing of colours, as a non-perfect mixing results in coloured edges and shadows, while white surfaces will not be white, but coloured.
Light guides are used in many lighting applications, such as for general-purpose lighting or as a backlight source for an LCD monitor or television. One such example is disclosed in International Patent Application WO 2004/008023 A1 in which a light guide device comprising an LED group as a light source, a light guide plate for colour mixing and a light guide plate having a light output face is shown. The light guide device is provided as backlight source behind a liquid crystal display panel. It further shows a triangular prism for guiding light rays from the LED group to one end face of the light guide plate and triangular prisms for guiding light rays from the other end face of the light guide plate to one end face of the light guide plate.
Another possible application in which these light guides can be used is for instance in LED based ambient background lighting for television sets, such as a flat screen display panel. Light effects are generated around the TV that matches the image contents. The effect gives the impression of a larger virtual screen and a more immerse viewing experience. In addition, it reduces the strain on the eyes of the viewers.
In order to achieve this ambient background lighting effect it is known to have a lamp positioned behind a television set, which lamp emits light towards the wall. For these purposes it is common to use CCFL lamps. For several reasons, a LED based version would be preferred. There are however known issues with colour mixing and colour uniformity associated with the use of LEDs. For instance, in order to be able to generate all colours, at least three different LEDs are required, i.e. one red, one green and one blue LED. The three LEDs are inherently positioned next to each other, and because of the different positions, the individual colours will not perfectly overlap each other in the output.
Using more LEDs per primary colour is an alternative, yet this substantially increases the cost of a solution. Hence it would be preferably to have only a few, or even only one LED per colour.
Another common technique that is used is to optically position the 3 LEDs on top of each other, by means of dichroic filters. Thus one imaginary source can be created that can generate all colours. However, also this solution has serious drawbacks. The main disadvantages of dichroic filters are the cost and the difficult methods of producing these. Furthermore they are strongly angle-dependent; light striking them under a different angle than intended might be transmitted instead of reflected, or vice-versa. Also, it is very difficult to include such filters inside a light guide, which in its simplest form consists of a single piece of glass or transparent plastic (e.g. Polymethylmethacrylate (PMMA)).
In order to achieve a more uniform distribution of light comprised of several colours, it is a common technique to use light guides with the characteristic of diffusing light. For instance, with total internal reflection (TIR) light can be held inside the light guide. Only when light encounters a surface with an angle sufficiently close to the normal, light may exit the light guide. However, due to various drawbacks, also this technique is in need for improvements.